Method and apparatus for producing free-standing silicon carbide articles

ABSTRACT

A process of producing relatively large, dense, free-standing silicon carbide articles by chemical vapor deposition is enabled by the provision of specially designed isolation devices. These devices segregate silicon carbide deposits on the intended portions of substrates, thereby alleviating the need to fracture heavy silicon carbide deposits in order to remove, or otherwise move, the substrate, with the heavy deposit thereon, from the deposition furnace. The isolation devices enable the use of more efficient vertically extended vacuum furnaces. The isolation devices also enable the commercial production of relatively dense, large, thin-walled, silicon carbide shells.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] Silicon carbide's unique combination of properties make it aparticularly suitable material for a variety of applications in thesemiconductor, optical, electronic and chemical processing fields.Moreover, chemical vapor deposition (CVD) techniques have been widelyused to provide thin films and coatings of a variety of materials onvarious articles. Silicon carbide articles produced by chemical vapordeposition (CVD) processing are recognized to exhibit superiormechanical, thermal, physical and optical properties. This invention isdirected to improvements in a CVD process of producing free standing,self-supporting silicon carbide articles, and is particularly adapted tothe production of hollow shells of cylindrical, frustoconical or othershapes. Such shells can be used in x-ray telescopes, semiconductorprocessing furnaces, heat exchangers, laser tubes and chemical processequipment.

[0003] 2. Description of Related Art

[0004] The advantages of silicon carbide as a fabrication material forastronomical X-ray telescopes and the experimental use of small scaleCVD processing to prepare conical silicon carbide shells was recentlydescribed by Geril et al. in “Thin Shell Replication of Grazing Incident(Wolter Type I) SiC Mirrors”, SPIE Proc., 2478, 215 (1995).

[0005] The advantages of CVD produced free-standing silicon carbidematerials in applications requiring a high degree of surface smoothnessand polishability are described in U.S. Pat. No. 5,374,412. The patentdescribes apparatus and process conditions which are used in the CVDproduction of free-standing silicon carbide articles. This patent alsorefers to earlier U.S. Pat. Nos. 4,990,374; 4,997,678 and 5,071,596 asfurther describing CVD processes of producing free-standing siliconcarbide materials by the pyrolytic deposit of SiC on a mandrel.

[0006] Several methods of controlling or isolating the deposit ofsilicon carbide to the intended side of the substrate during chemicalvapor deposition are described in U.S. Pat. Nos. 4,963,393 and4,990,374. In Pat. No. 4,963,393, a curtain of a flexible graphite clothis arranged to shield the backside of the substrate from the flowingreacted precursor gases, whereby silicon carbide deposits on thebackside of the substrate are avoided. In U.S. Pat. No. 4,990,374 acounterflow of a non-reactive gas is directed to flow past thesubstrate's peripheral edge from behind the substrate whereby thedeposit is confined to the front face of the substrate.

SUMMARY OF THE INVENTION

[0007] Chemical vapor deposition (CVD) has been used to produce bothfree-standing articles and coatings of silicon carbide. Typically, theprocess involves reacting vaporized or gaseous chemical precursors inthe vicinity of a substrate to result in silicon carbide depositing onthe substrate. The deposition reaction is continued until the depositreaches the desired thickness. If a coated article is desired, thesubstrate is the article to be coated and the coating is relativelythin. If a free-standing article or silicon carbide bulk material isdesired, a thicker deposit is formed as a shell on the substrate andthen separated from the substrate to provide the silicon carbidearticle.

[0008] In a typical silicon carbide bulk material production run,silicon carbide precursor gases or vapors are fed to a depositionchamber where they are heated to a temperature at which they react toproduce silicon carbide. The silicon carbide deposits as a shell on asolid mandrel or other substrate provided in the deposition chamber. Thedeposition is continued until the desired thickness of silicon carbideis deposited on the substrate, or mandrel. The mandrel is then removedfrom the deposition chamber and the shell is separated from the mandrel.Monolithic silicon carbide plates and cylinders have been produced byapplying such chemical vapor deposition (CVD) techniques with suitablyshaped substrate or mandrel forms.

[0009] Once the silicon carbide precursor gases or vapors are brought tothe appropriate conditions to cause them to react, they produce siliconcarbide which then deposits on any available surface. The depositgenerally is not limited to the intended surface(s) of the mandrel(s)and generally extends past such surfaces to adjoining surfaces as wellas depositing on the walls and housing of the deposition chamber. In thepast, the silicon carbide deposit has extended past the dimensionallimits of the mandrel over adjacent portions of the support structureholding or supporting the mandrel in its position in the depositionchamber. It is then necessary to fracture such deposits to remove themandrel from the deposition chamber. Fracturing of the deposit oftenresults in the formation of cracks which propagate through the depositfrom the point of fracture. Such cracks are not acceptable in theintended applications of the silicon carbide articles, and usuallyresult in the article being rejected. The prevalence of propagatedcracks in relatively thick chemical vapor deposits of silicon carbidehave limited the size of articles that can be produced commercially bythis method. Moreover, recognition of the potential capacity of CVDsilicon carbide deposits to bridge any joints between adjacent stackedmandrels and the subsequent difficulty of separating and removingindividual mandrels from such a stack has prevented the use of stackedmandrels in the commercial production of silicon carbide articles.

[0010] Optimal deposition conditions generally require less thanatmospheric pressures, which requires that the deposition be conductedin a vacuum chamber. It is generally less expensive to increase theproduction volume of vacuum chambers by increasing their verticaldimensions rather than increasing their horizontal, or floor spaceoccupying, dimensions. Accordingly, it would be economicallyadvantageous to provide a commercial technique for creating siliconcarbide deposits on a plurality of mandrels, wherein the mandrels arevertically stacked within a single vertically extending depositionchamber. This, however, has not been done in the past, at least in partbecause of the difficulty in segregating, or isolating, the deposit onone mandrel from the deposit produced on an adjoining mandrel.

[0011] The present invention is directed to a process, and associatedapparatus, which greatly restricts and, preferably, completely avoids,the formation of deposits extending past the dimensional limits of themandrel. By limiting, or avoiding, the formation of such deposits,removal of the mandrel from the deposition chamber does not result incracks which propagate through the deposit. When practice of theinvention avoids the formation of a deposit at or adjacent thedimensional boundary of the mandrel, the mandrel can be removed from thedeposition chamber without fracturing the deposit. When a greatlyrestricted deposit forms at the dimensional boundary of the mandrel, itforms a thin coating, substantially thinner than the main body of thedeposit, the fracture of which does not result in cracks extending intothe main body of the deposit.

[0012] The present invention also provides a process wherein siliconcarbide deposits are formed on a plurality of substrates, or mandrels,as they are arranged in a vertical stack, one atop another. The mandrelsare then removed from the stack and the deposits separated from themandrels to result in free-standing dense silicon carbide articles.

[0013] The invention further provides for the production of rigid,thin-walled cylindrical or frustroconical shells of dense siliconcarbide having an aspect ratio, i.e., the ratio of the shell diameter toits wall thickness, of 50 or greater. It also has permitted thecommercial production of large diameter, i.e. 18 inch diameter andgreater, cylindrical or frustoconical shells of dense silicon carbide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic illustration, partially in section, of afurnace utilizing the process and apparatus of the present invention toproduce an inventive relatively dense, thin-walled, large siliconcarbide shell.

[0015]FIG. 2 is a cross section of one type of isolation deviceaccording to the present invention, deployed in the CVD furnace with adeposit on a mandrel substrate.

[0016]FIG. 3 is a further cross section of the mandrel and isolationdevice illustrated in FIG. 2.

[0017]FIG. 4 is a cross section of a further type of isolation deviceaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] A chemical vapor deposition (CVD) furnace equipped for producingsilicon carbide shells according to the present invention is illustratedin FIG. 1. The furnace 10 includes an outer housing defining a vacuumchamber 12, which is connected to a vacuum source through exhaust port14. A deposition chamber 16, provided in the vacuum chamber, is definedby side wall 18, top wall 20 and base 22. The base 22 is supported bycolumns 24 extending up from a floor plate 26 provided near the bottomof the vacuum chamber 12. A rotating platform 28 is supported by rails30 which extend into channels 32 on table 34. Shaft 36 transmits amotive force to rotate the platform 28 from a motor/transmission system(not shown). An arrangement of gas injectors 38 feed the depositprecursor gases through the top wall 20 into the deposition chamber 16.Typically, the arrangement of injectors 38 involves multiple injectorsarranged around a central injector. A stack of two mandrels 40 and 42 isarranged on the rotating platform. Each of the mandrels comprise a sidewall 44 formed from a hollow conical graphite form which extends from asmall end 46 to a large end 48. The diameter of the mandrel at the smallend 46 is less than the diameter at its large end 48. As arranged inFIG. 1, the mandrels are stacked with their large ends 48 adjacent eachother. Suitable heating means (not illustrated) provide the desireddeposition temperature at the mandrel surface. Graphite electricalresistance heaters arranged along the side wall and beneath the bottomof the mandrels have provided relatively uniform temperaturedistribution over the mandrel surface.

[0019] In a typical silicon carbide production run, a single mandrel 42is located on the rotating platform 28, the deposition chamber 16assembled and the vacuum chamber 12 closed. The vacuum chamber is purgedof atmospheric gases by drawing a vacuum on the chamber, injecting aninert gas through the gas injectors 38, and redrawing a vacuum. Thesesteps are repeated until the atmospheric gases are adequately purged.The rotation of the platform 28 is then initiated, the mandrels heatedto the target deposition temperature, the flow of reactive coatingprecursor gas initiated and the target furnace pressure established. Theflow of precursor gas, the target mandrel temperature and the targetfurnace pressure are continued until the deposit reaches the desiredthickness, at which time the flow of coating precursor gas isdiscontinued, and the mandrel temperature and furnace pressure allowedto return to normal, or ambient. The vacuum chamber is then opened(usually from the bottom), the rotating platform lowered, and themandrel removed therefrom. The silicon carbide deposit is then separatedand recovered from the mandrel. In the past, removal of the mandrel fromthe rotating platform was complicated by the silicon carbide depositwhich not only formed on the mandrel, but extended past the end of themandrel along the top surface of the rotating platform 28. Removal ofthe mandrel required fracturing this relatively thick deposit whichcreated cracks, which, in turn, could propagate from the point offracture throughout the deposit, in many cases precluding the use of thesilicon carbide article for its intended use.

[0020] In the present process an isolation device 50 is provided betweenthe bottom mandrel 42 and the rotating platform 28. As best seen in FIG.2, the isolation device 50 includes a side or outer wall portion 52 anda closed end portion 54. The outer wall portion 52 is spaced from themandrel 42 and extends from the closed end portion 54 to an open end 56.The closed end portion extends between the mandrel 42 and the outer wallportion 52. Together, the outer wall portion 52, the closed end portion54 and the mandrel 42 define an open channel 58 which extends from theclosed end to the open end 56. As best seen in FIG. 3, at its open end56, the width of the open channel, w₁, or the distance between the outerwall portion 52 and the mandrel 42, is one to two times the intendedthickness of the final silicon carbide deposit 57. Preferably, the widthof the channel, w₂, at the closed end 54 is less than ½ of the width w₁.The provision of a smaller channel width, w₂, at the closed end assuresthat no deposit will form on the mandrel where it adjoins the closed end54. The height, h, of the open channel 58 from the closed end 54 to theopen end 56 is greater than the width w₁, and, preferably, isapproximately 1.5 to 5 times the width w₁.

[0021] The open channel 58 defines a boundary zone 60 on that portion ofthe surface of the mandrel, or substrate, which lies adjacent the openchannel, i.e. that portion of the mandrel extending beneath the outerwall portion 52 of the isolation device 50. The boundary zone isdistinguished from the remaining portion of the mandrel surface, whichportion is referred to as the deposition zone 62, by its diminishedavailability to the reacting coating precursor gases, resulting in theformation of a substantially thinner silicon carbide deposit in theboundary zone. The deposit thickness within the boundary zone decreasesthe further it is from the channel's open end 56. Preferably, thedeposit thickness decreases sufficiently that essentially no deposit isformed adjacent the closed end 54 of the channel.

[0022] Utilization of the isolation device 50 between the mandrel 42 andthe platform 28 in the inventive process provides a boundary zone 60 ofdiminished deposit thickness between the deposition zone 62 and thatportion of the mandrel which is most proximate the supporting solidsurface of the rotating platform 28. Preferably, the deposit thicknessdiminishes to essentially zero at the end of the mandrel most proximateto the platform 28. Accordingly, removal of the mandrel 42 with thesilicon carbide deposit thereon from the isolation device 50 does notrequire fracturing a thick extension of the deposit and the accompanyingpossibility of creating propagating cracks in the product article. Theyield of acceptable product is thereby substantially enhanced byminimizing or eliminating the silicon carbide deposit where theremovable mandrel adjoins non-removable components of the furnace.

[0023] An isolation device 64, which is essentially identical to theisolation device 50, is provided at the top of the upper mandrel 40where it serves to prevent the silicon carbide deposit from extendingover the upper rim and onto the interior surface of the hollow uppermandrel 40. The closed end portion of isolation device 64, like that ofdevice 50, is a dish-like continuous solid sheet spanning the spacedefined by the outer wall. Accordingly, the isolation device 64 alsofunctions to essentially close the upper end of the stack therebydenying the reactive precursor gases access to the interior of the stackof hollow mandrels 40 and 42 and avoiding unwanted deposits forming onthe interior surfaces of the hollow mandrels.

[0024] A further isolation device 66 is provided between the stackedmandrels 40 and 42. The isolation devices 50 and 64 are designed tocooperate with a single mandrel in defining a boundary zone adjacent theperiphery of such mandrel. Isolation device 66 is designed to cooperatewith two stacked mandrels in providing isolation zones on each. As seenbest in FIG. 4, isolation device 66 comprises a circular ring comprisinga circumferentially extending outer wall portion 70 with a radiallydisposed closed end portion 72 extending inwardly from the center of theinner face of the outer wall portion so as to result in the ring havinga generally T-shaped cross-section. As deployed, the radially disposedclosed end portion 72 is located on top of the upper edge of the lowermandrel 42 and the lower edge of the upper mandrel 40 is located on topof the closed end portion 72. The circumferentially extending outer wallportion 70 extends around both lower and upper mandrels functioning witheach of them and the closed end portion 72 to define boundary zones 60at each of the mandrels' adjoining edges. During the deposition processthe isolation device restricts the flow of reacting precursor gases tothese boundary zones whereby the thickness of the deposit formed in eachof the boundary zones gradually decreases, preferably to zero, as thedeposit approaches the closed end. Since the deposit is essentiallycompleted before reaching the closed end and does not extend across thejunction of the mandrel with the isolation device, the separation of themandrel from the isolation device at the completion of the run does notproduce cracks which propagate throughout the deposit. In the isolationdevice illustrated in FIG. 4, the radially disposed closed end portion72 is sufficiently long to be fully supported by the lower mandrel 42and to provide full support for the upper mandrel 40, but does notextend from one side to the other, as do the illustrated isolationdevices 50 and 64. While the device 66 could be designed to completelyisolate the interiors of the adjacent mandrels, a savings in weight,material and cost is achieved by providing a device 66 with theillustrated annular closed end portion 72. The closed end portion ofdevice 66 is thicker adjacent its outer end portion 70 than it is whereit contacts the mandrel. The change in thickness occurs at a step 74located at a diameter equal to the external diameter of the associatedmandrel plus two times the intended w₂ dimension, i.e. the intendedwidth at the closed end of the channel.

[0025] One of ordinary skill will recognize that the upper and lowermandrels may have differing length, width and thickness dimensions. Thedimensions of the isolation devices can be readily determined from thedimensions of the particular mandrels to be isolated.

[0026] A vertical stack of two mandrels is illustrated in the FIG. 1embodiment. The stack can include 4, 6, or any number of mandrels,provided they are separated with isolation devices at each junction. Thecapability of processing multiple mandrels in a vertical stack enablesthe process to be conducted in vertically oriented vacuum furnaces whichgenerally require less floor space, less capital and less maintenanceexpense than horizontally oriented vacuum furnaces of the same capacity.

[0027] The substrates, or mandrels, are typically shaped around hollowcores to minimize their cost and weight. The mandrels can have generallycircular or annular cross-sections, as in cylindrical or frustroconicalmandrels. Moreover, the mandrels may incorporate several distinct shapesas they extend along their axial lengths. Double frustroconical shapedmandrels in which two cones having different side wall angles convergein the middle portion of the mandrel have been used. Alternatively, whenflat sheets of silicon carbide are to be produced, the mandrel maycomprise a series of connected planar walls extending around a hollowcore, like the four side walls of a box.

[0028] The mandrels are fabricated from appropriate high temperaturematerials such as alumina, graphite, molybdenum or tungsten. Graphite isgenerally preferred because of its close thermal expansion match withsilicon carbide, its high temperature properties and its availability inlarge sizes. The SiC-12 grade of graphite produced by Toyo Tanzo Inc. isparticularly preferred when the deposit is formed on the exterior orperipheral surface of the mandrel. The thermal expansion coefficient ofthis grade of graphite is just slightly greater than silicon carbide'scoefficient, assuring that the mandrel will shrink slightly more thanthe deposit during cool down. When graphite mandrels are used,separation of the deposit from the mandrel is usually accomplished bycombustion of, or burning away, the graphite at a temperature between600° and 800° C. The isolation devices can be fabricated from similarmaterials. Usually less expensive grades, i.e. grades which mightotherwise form small cracks at the deposition conditions, of thesematerials can be used in the isolation devices since they do not serveto shape the product article.

[0029] The deposition process may produce the intended article as adeposit on the exterior of the mandrel, as illustrated, or it mayproduce the product deposit on the interior of a hollow substrate. Thedeposit is usually machined to its final dimensions following itsremoval from the mandrel. However, when it is not intended to machinethe removed deposit, the surface of the article with the more criticalsurface dimensions is usually formed directly adjacent the substrate.

[0030] A mold release coating may advantageously be applied to thesubstrate surface prior to initiating the deposition, particularly whenlarge sized articles are deposited. Amorphous, glassy or pyrolyticcarbons are suitable release agents for use with graphite mandrels.

[0031] The CVD production of bulk, or free-standing, silicon carbidearticles involves feeding a mixture of silicon carbide precursor gases,such as a mixture of methyltrichlorosilane (MTS) and H2, with anoptional inert gas, such as argon, helium or nitrogen, to the heatedreactor/deposition chamber which is maintained at a pressure betweenabout 180 and 220 torr, and at a temperature between about 1340 and1380° C. The mandrel(s) is rotated at a speed in the range of 1 to 5rpm. The relative partial pressure flow ratio of H₂/MTS is maintained inthe range of about 4 to about 10. Silicon carbide is deposited on themandrel(s) at a deposition rate of about 1.0 to about 2.0 μm/min. and iscontinued until the desired thickness of SiC is deposited. Any desiredthickness can be produced by merely continuing the deposition forsufficient time, however, relatively thin-walled shells are generallydesirable based on weight, cost and other considerations.

[0032] After the mandrel with the deposit is removed from the rotatingplatform, the mandrel with the deposit thereon may be cut to therequired length and the outer surface of the deposit machined. Themandrel is then removed by burning the graphite. If necessary, theinside surface of the deposit can then be machined to its requiredspecification.

EXAMPLE

[0033] Two graphite shell mandrels, fabricated from SiC-12 grade ofgraphite, were machined to final dimensions of 600-mm diameter and240-mm length. They were then stacked in a CVD-furnace similar to thatshown in FIG. 1. Isolation devices wherein the dimensions shown in FIG.3 were a w₁ of approximately {fraction (9/16)}th of an inch, a height,h, of approximately one inch and a w₂ of approximately {fraction(3/16)}th of an inch. A reactive precursor gas was injected through anarray of seven injectors, six equally spaced in an approx. 36 inchdiameter circle and one located in the circle's center. The precursorgas mixture was provided at a flow rate through each injector ofmethyltrichlorosilane 4.4 standard liters per minute (slpm), H₂ 22 slpm,and Ar 56.5s lpm. The mandrels were rotated at a speed of 1.5 rpm andwere maintained at a target temperature of 1350° C. for 90 hours. Thedeposits provided on the deposition zones of the two mandrels variedbetween 0.149 and 0.348 inches in the axial direction. The thicknessvariation in the radial direction was within 7%. The furnace was openedand the mandrels and isolation devices removed without introducingpropagating cracks throughout the deposits. The deposits were then cutto the specification length of 240-mm and the outer surfaces machined tospecification. The graphite mandrels were then removed by burning themandrels at temperatures in the 600-800° C. range. The interior surfaceof a SiC shell was then machined to provide a finished shell of 600-mmdiameter by 240-mm length by 3-mm wall thickness.

[0034] The invention permits the commercial fabrication of relativelydense, large-diameter, thin-walled silicon carbide cylindrical andfrustroconical shells or tubes. Chemical vapor deposition techniquesprovide deposits of 3.15 g/cc and greater densities, which correspond toat least 98% theoretical density. Use of the isolation devices duringCVD processing avoids, or at least minimizes the formation of propagatedcracks throughout the deposit which previously had precluded thepreparation of hollow shells of 18 inch or larger diameters (i.e.,shells having external perimeters of 60 inches or greater). Asillustrated in the preceding example a shell of 24 inch diameter and3-mm wall thickness and having an aspect ratio (shell diameter/ wallthickness) of approximately 203 was prepared by this method. Theinvention encompasses shells of dense silicon carbide having externalperimeters (i.e., circumferences) of 50 inches or greater, andparticularly, those having external perimeters of 65 inches or greater;and having aspect ratios of 50 or greater; preferably, shells havingaspect ratios of 100 or greater; and most preferably, those havingaspect ratios of 200 or greater.

[0035] The foregoing is provided to enable workers in the art topractice the invention, and to describe what is presently considered thebest mode of practicing the invention. The scope of the invention isdefined by the following claims.

We claim:
 1. In a process for producing silicon carbide articles bychemical vapor deposition comprising: providing a silicon carbideprecursor gas in proximity to a surface of a solid substrate in adeposition chamber, reacting said silicon carbide precursor gas toprovide a silicon carbide deposit on a predetermined deposition zone onsaid surface of said substrate, thereafter removing said substrate withsaid silicon carbide deposit from said deposition chamber, andrecovering said deposit by separating it from said substrate, theimprovement comprising: providing at least one boundary zone on aportion of said surface located between said predetermined depositionzone and a proximate solid surface in said deposition chamber, andproducing a silicon carbide deposit on the predetermined deposition zonewhich is substantially thicker than the deposit produced in saidboundary zone.
 2. The process of claim 1, wherein the thickness of saiddeposit produced in said boundary zone decreases as it extends away fromsaid deposition zone.
 3. The process of claim 1, wherein essentially nodeposit is formed on the portion of said boundary zone which is closestto said proximate solid surface.
 4. The process of claim 1, wherein anisolation device is arranged between said proximate solid surface andsaid substrate, and said isolation device includes a side wall whichextends over said boundary zone.
 5. The process of claim 4 wherein saidproximate solid surface is a surface of a second solid substrate.
 6. Theprocess of claim 4, wherein said proximate solid surface supports saidisolation device and said isolation device supports said substrate.
 7. Aprocess of producing a silicon -carbide article, comprising: providing asilicon carbide precursor gas in proximity to a predetermined depositionzone on a surface of a solid substrate in a deposition chamber, reactingsaid silicon carbide precursor gas to form a silicon carbide deposit onsaid predetermined deposition zone, defining a boundary zone ofdiminished deposit thickness on said surface adjacent said predetermineddeposition zone, providing a channel overlying said boundary zone, saidchannel being defined by (a) said boundary zone, (b) an outer wallspaced from and extending over said boundary zone, (c) a closed endextending between said boundary zone and said outer wall, and (d) anopen end opposite said closed end and adjacent said deposition zone, andrecovering said silicon carbide deposit from said substrate surface. 8.The process of claim 7, wherein the width of said channel at its openend (w₁) is one to two times the thickness of the recovered deposit. 9.The process of claim 7, wherein the distance between the channel's openend and its closed end (h) is 1,5 to 5 times the width of said channelat its open end (w₁).
 10. The process of claim 7, wherein the width ofsaid channel at its open end (w₁) is at least twice its width at itsclosed end (w₂).
 11. The process of claim 7, wherein said substrate isseparated from another solid surface in said deposition chamber by anisolation device.
 12. The process of claim 11, wherein said outer walland said closed end are integral parts of said isolation device.
 13. Theprocess of claim 12, wherein said substrate is supported by saidisolation device.
 14. The process of claim 13, wherein said isolationdevice is supported by said another solid surface.
 15. The process ofclaim 12, wherein said isolation device separates two substrates. 16.The process of claim 7, wherein said substrate extends around a hollowcore.
 17. The process of claim 16, wherein said substrate has acylindrical or frustroconical shape.
 18. The process of claim 16,wherein said substrate comprises a series of planar walls extendingaround said hollow core.
 19. The process of claim 16, wherein said outerwall and said closed end are integral parts of an isolation device whichseparates the interior hollow core of said substrate from said precursorsilicon carbide gas in said deposition chamber.
 20. An apparatus forforming solid deposits from gaseous precursors, comprising: a solidsubstrate, a housing defining a deposition chamber, said housing beingcapable of opening and closing sufficiently to allow the insertion andremoval of said solid substrate, a source of a gaseous precursormaterial operatively connected to said deposition chamber, an isolationdevice located between said solid substrate and a proximate solidsurface in said deposition chamber, said isolation device cooperatingwith said substrate to restrict the flow of said gaseous precursormaterial over a boundary zone extending adjacent the border of saidsubstrate closest to said proximate solid surface.
 21. An apparatusaccording to claim 20, wherein: said isolation device comprises an outerwall spaced from and extending over said boundary zone from an open endto a closed end, said closed end extending between said substrate andsaid outer wall.
 22. An apparatus according to claim 21, wherein: saidouter wall is spaced a greater distance from said solid substrate atsaid open end (w₁) than it is spaced from said solid substrate at saidclosed end (w₂).
 23. An apparatus according to claim 21, wherein: saidopen end of said outer wall is spaced from said closed end a distancewhich is 2 to 5 times the distance, w₁, the outer wall portion is spacedfrom said substrate at said open end.
 24. An apparatus according toclaim 21, wherein: two solid substrates are arranged one atop the otherin said deposition chamber, and said isolation device is located betweenthe adjacent solid surfaces of the two substrates.
 25. An apparatusaccording to claim 24, wherein: said substrates are generallycylindrical or frustroconical in shape.
 26. An apparatus according toclaim 20, wherein: said substrate is supported in said depositionchamber by said isolation device.
 27. A hollow silicon carbide shellhaving a ratio of external perimeter to wall thickness greater than 50.28. The hollow shell of claim 27 having a cylindrical shape.
 29. Thehollow shell of claim 27 having a frustroconical shape.
 30. The hollowshell of claim 27, wherein the density of said silicon carbide is atleast 3.15 grams per cubic centimeter.
 31. The hollow shell of claim 27,wherein said external perimeter is in excess of 50 inches.
 32. Thehollow shell of claim 27, wherein said external perimeter is in excessof 65 inches.
 33. The hollow shell of claim 27, wherein said ratio is200 or greater.